SECTION III 



THE REGULATION OF THE RESPIRATORY 

 MOVEMENTS 



EACH movement of inspiration involves the co-ordinated activity of a large 

 number of muscles. Thus the diaphragm and the intercostal muscles 

 must come into action at the same time, and the extent to which they 

 contract will determine the depth of the inspiration. Similarly, they must 

 cease to act simultaneously if the act of expiration is to take place. The 

 rhythm and extent of the alternate contractions and relaxations of the 

 respiratory muscles are determined, as we have seen, by the needs of the 

 organism as a whole. These respiratory movements are regulated so that 

 the total ventilation of the alveoli shall be sufficient to meet the gaseous 

 exchanges of the body. Whether the organism consumes 250 or 1000 c.c. of 

 oxygen per minute, the respiratory movements keep the composition of the 

 gas in the alveoli at a practically constant level. 



The muscles involved both in inspiration and expiration can be thrown 

 into activity only by the intermediation of nerves. Each act of inspiration 

 involves a discharge along a number of nerves, e. g. the facial to the muscles 

 moving the alae nasi, the vagus to the muscles of the larynx, the branches 

 of the cervical and brachial nerves to the muscles of the neck, the phrenic 

 nerves to the diaphragm, and the dorsal nerves to the intercostal muscles. 

 The fibres making up these nerves are derived from nerve cells of the anterior 

 horn, situated at various levels in the medulla and spinal cord. In each act 

 of inspiration or expiration the activities of all these groups of cells must be 

 brought into relation among themselves, as well as with the needs of the 

 organism for oxygen and for the elimination of carbon dioxide. It is 

 conceivable that the co-ordination of the activities of the various motor 

 nuclei might be attained by the provision of communicating nerve paths 

 joining the centres among themselves, and by a sensibility of all these centres 

 to the gaseous contents of the blood as well as to the influence of afferent 

 .impressions from the periphery. A much more efficient co-ordination 

 however would be effected by the subjection of these motor nuclei to the 

 action of some specialised portion of the central nervous system, which 

 would act as a receiving centre for afferent impressions from the lungs and 

 surface of the body, and would be endowed with a special sensibility to 

 changes in the composition of the blood circulating through its vessels. 

 Experiment shows that the latter method is employed in the organism for 

 the regulation of the respiratory movements. If the spinal cord be cut across 



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